Influence of Cation Substitution and Activator Site Exchange on the

Nov 14, 2013 - Fax: + 91 471 2491712. ... The Eu3+ emission due to magnetic dipole transition (5D0–7F1 MD) is .... based pyrochlore type red phospho...
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Influence of Cation Substitution and Activator Site Exchange on the Photoluminescence Properties of Eu3+-Doped Quaternary Pyrochlore Oxides S. K. Mahesh, P. Prabhakar Rao,* Mariyam Thomas, T. Linda Francis, and Peter Koshy Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology(NIIST), CSIR, Trivandrum 695 019, India S Supporting Information *

ABSTRACT: Stannate-based pyrochlore-type red phosphors CaGd 1 − x SnNbO 7 :xEu 3 + , Ca 1 − y Sr y Gd 1 − x SnNbO 7 :xEu 3 + , and Ca0.8−xSr0.2GdSnNbO7+δ: xEu3+ were prepared via conventional solidstate method. Influence of cation substitution and activator site control on the photoluminescence properties of these phosphors are elucidated using powder X-ray diffraction, Rietveld analysis, Raman spectrum analysis, and photoluminescence excitation and emission spectra. The Eu3+ luminescence in quaternary pyrochlore lattice exemplifies as a very good structural probe for the detection of short-range disorder in the lattice, which otherwise is not detected by normal powder X-ray diffraction technique. The Eu3+ emission due to magnetic dipole transition (5D0−7F1 MD) is modified with the increase in europium concentration in the quaternary pyrochlore red phosphors. 5D0−7F1 MD transition splitting is not observable for low Eu3+ doping because of the short-range disorder in the pyrochlore lattice. Appearance of narrow peaks in Raman spectra confirms that short-range disorder in the crystal lattice disappears with progressive europium doping. By using Sr as a network modifier ion in place of Ca we were able to increase the f−f transition intensities and europium quenching concentration. The influence of effective positive charge of the central Eu3+ ions when it replaces a metal ion having lower oxidation state such as Ca2+ was also investigated. The relative intensities of A1g (∼500 cm−1) and F2g (∼330 cm−1) Raman vibrational modes get inverted when Eu3+ ions replaces Ca2+ ions instead of Gd3+ as trivalent europium ions can attract the electron cloud of oxygen ions strongly in comparison with divalent calcium ions. The influence of positive charge effect of Eu3+ in Ca0.7Sr0.2GdSnNbO7+δ:0.1Eu3+ phosphor is greatly strengthened the charge transfer band and 7F0-5L6 transition intensities than that of the Ca0.8Sr0.2Gd0.9SnNbO7:0.1Eu3+ phosphor. Our results suggest that the photoluminescence properties can be enhanced by simple compositional adjustments in the quaternary pyrochlore-type red phosphors. ultraviolet region found applications in the field of plasma display panels, field-emission displays, high-pressure Hg discharge lamps, etc.11 But, highly efficient, low-powerconsuming, solid-state lighting technology requires phosphors that are excitable under near UV(∼395 nm) or blue light (∼465 nm). Eu3+-doped red phosphors attained much attention in the area of phosphor converted white-lightemitting diodes because they exhibit a high lumen equivalent, quantum efficiency, and photostability at the same time.12 Pyrochlore compounds with a general formula A2B2O7 offer good qualities of a host lattice such as high chemical stability, lattice stiffness, thermal stability, etc, and at the same time allows a wide variety of chemical substitutions at the A and B sites (3+ and 4+ or 2+ and 5+ combinations of valence, as well as oxygen vacancies).13 The unit cell of this structure is usually face-centered cubic with a space group Fd3m ̅ with eight formula units per unit cell (Z = 8). The A site (16d) cation is in 8-fold

1. INTRODUCTION Among the rare-earth-doped phosphors, Eu3+ luminescence has attained great importance because of its simple electronic energy level scheme and the hypersensitive transitions that can be used as a probe for site symmetry studies,1,2 structure− luminescence correlations,3,4 covalency/polarizabilty effect of ligands,5 chemical bond differences in solids,6 etc. Eu3+ emission spectra consist of a series of sharp lines arising from intraconfigurational 5D0-7 FJ transitions in which 5 D0 -7F 1 magnetic dipole (MD) transition around 590 nm or 5D0-7F2 electric dipole (ED) transition around 612 nm dominates depending on the site symmetry of Eu3+ sites. When Eu3+ locates in a site with inversion symmetry MD transitions dominates otherwise the ED transition.7 Splitting of these transitions give an insight into the crystal field effect at the activator sites.8,9 The appearance of 5D0-7F0 transition of Eu3+ is an indication of lowering of Eu3+ crystal site symmetry as it is allowed only for Cs, Cn, Cnv site symmetries and it shows a red shift with the increase in covalency of Eu3+.1,10 Eu3+-doped phosphor materials that can be excited in the vacuum © 2013 American Chemical Society

Received: March 7, 2013 Published: November 14, 2013 13304

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of the absence of suitable host system.24 The present quaternary pyrochlore CaGdSnNbO7 offers such kind of studies as the Eu3+ ion can be doped to Ca2+/Gd3+ ions without any significant change in the structure/electronegativity of ligands and it was found that doping of Eu3+ ions to the Ca2+ ion site leads to some drastic increase in the f−f excitation peaks and in emission intensity, which is somewhat interesting because in most cases, better luminescence properties are observed for isovalent substitution. The luminescence emission changes with respect to bigger cation substitution and aliovalent doping is also discussed in detail in this work.

coordination with oxygen anions, whereas the smaller B site (16c) cation resides in 6-fold coordination forming a (BO6) oxygen octahedra. Cations having higher ionic radii (∼1 Å) occupy the A-site, and the lower one (∼0.6 Å) goes to the B sites. In pyrochlore phosphors, Eu3+ (ionic radius 1.067 Å, CN8) is expected to occupy A-site having site symmetry D3d. The inversion symmetry of the pyrochlore A site makes it difficult to get narrow red emission band through ED transition by Eu3+ doping. Though red emitting pyrochlore phosphors are reported,14−16 most of the Eu3+-doped pyrochlore phosphors give MD dominated orange emission.17−19 The oxygen has two distinct sites: oxygen ions on the 48f site ,which are bonded tetrahedrally to two A and two B cations ,and those on 8b sites (O′) that are bonded tetrahedrally to four A cations (8a site is vacant in an ideal pyrochlore structure taking B as origin, this vacant 8a site is coordinated by four B cations). All the A and B cations and oxygen ions in the 8b sites are on special positions but the 48f oxygen position varies. The A and B site coordination polyhedra are joined along edges, and the shapes of these polyhedra change as the positional parameter ‘x’ of the O48f shifts to accommodate cations of different sizes. These two polyhedra have D3d symmetry. For x = 0.3750, the A site coordination polyhedron is a regular cube, and the B site polyhedron is distorted to a trigonally flattened octahedron. In this case, materials have a defect fluorite structure and occupancy of each anion site is 0.875. For x = 0.3125, the B site is a regular octahedron and the A site is a distorted scalenohedron, and materials have the ideal pyrochlore structure. Thus, the 48f oxygen positional parameter, x defines the polyhedral distortion and structural deviation from the ideal fluorite structure. Yang et al. reported the Eu3+-doped luminescence properties of Gd2Sn2O7 phosphors.20 In the emission spectra of these phosphors, MD transition intensity dominates over ED transition because of the inversion symmetry of Eu3+ sites and the excitation spectra has weak f−f transition intensities in comparison with charge transfer (CT) band intensity. For an enhanced parity forbidden red emission in pyrochlore phosphors either we have to destruct the inversion symmetry of Eu3+ site or reduce the CT band energy such that there will be an enhanced probability of mixing of CT band wave function with f orbital and thereby a relaxation in the parity selection rule.7 Incorporation of cations with different valency and electronegativity to the crystal lattice can cause local distortion and modification in polarizability of chemical environment of Eu3+, which can change the photoluminescent properties.10,21−23 In the present study, we tried to enhance the f−f transition intensity of gadolinium stannate pyrochlore phosphors by incorporating Ca2+ ions to the A site and Nb5+ ions to the B site. Ordered pyrochlore compounds can have short-range disorder which is very difficult to detect from X-ray diffraction studies. Absence of 5D0-7F1 MD transition splitting for low europium doping concentration in CaGd1‑xSnNbO7:xEu3+ gives some information about shortrange disorder in the pyrochlore crystal lattice. The substitution of calcium ions by bigger strontium ions found to have influence on the concentration quenching. The influence of effective positive charge of the central Eu3+ ions when it replaces a metal ion having lower oxidation state is also a question of interest. Though it is reported that the effective positive charge of the central activator ion will shift the charge transfer band to lower energy region, its extent of influence on CT band and f−f excitation is very difficult to estimate because

2. EXPERIMENTAL SECTION The pyrochlore compounds CaGd1−xSnNbO7:xEu3+, Ca1−ySry Gd1−xSnNbO7:xEu3+ and Ca0.8−xSr0.2GdSnNbO7+δ:xEu3+ were synthesized through a solid state method using CaCO3, SrCO3, Gd2O3, Eu2O3, SnO2, and Nb2O5. The precursors are from the M/s Across Organics with 99.9% purity and the precursor oxides are fine powders. These precursors were taken in the required stoichiometric ratios and mixed thoroughly in an agate mortar using acetone as a wetting medium until fine slurry was obtained. The slurry was dried by placing it in an air oven at a temperature of 100 °C. This process of mixing and drying was repeated thrice to get a homogeneous product. These products were calcined at 1300−1400 °C for 6 h with intermittent grinding until a phase-pure compound was obtained. The electrical heating of the furnace is programmed initially 10 °C/min up to 900 °C and then onward 5 °C/min until the desired temperature, 1300−1400 °C. The calcined product was then ground into a fine powder for carrying further characterizations. CaGd1−xSnNbO7:xEu3+ and Ca1−ySryGd1−xSnNbO7:xEu3+ compounds were calcined at 1300 °C for 6 h. Ca0.8−xSr0.2GdSnNbO7+δ:xEu3+ phosphors give better luminescence properties when calcined at 1300 and 1400 °C for 6 h. X-ray powder diffraction (XRD) analysis was performed with a PANalytical X′Pert Pro diffractometer having Ni filtered Cu−Kα radiation with X-ray tube operating at 40 kV, 30 mA, and 2θ varied from 10 to 90° in 0.016° steps. Structure refinement of the XRD data was performed by Rietveld analysis using GSAS program.25 The obtained powder data was refined using the starting model of Gd2Sn2O7 with the origin placed on B site and the space group Fd3̅m (No. 227) of the pyrochlore structure. The bigger cations, Ca2+, Sr2+, Gd3+, and Eu3+ occupy the A sites at 16d (1/2, 1/2, 1/2) and the B ones, Sn4+ and Nb5+ are at 16c sites (0, 0, 0). There are two crystallographically nonequivalent sites for oxygen: one at the 48f sites (x, 1/8, 1/8) and the other at the 8b sites (3/8, 3/8, 3/8). First, the scale factors, zero correction, and background parameters were refined, the cell parameter, 48f oxygen positional parameter (x), full width halfmaximum (fwhm) parameters, and isotropic displacement factors being refined next. An asymmetry correction was also applied. The profile was fitted using a pseudo-Voigt profile function. Raman spectra were recorded using a HR800 LabRAM confocal Raman spectrometer operating at 20 mW laser power equipped with a peltier cooled CCD detector. Samples were excited using a He−Ne laser source having an excitation wavelength of 632.8 nm and with an acquisition time of 5 s using a 50× microscope objective. All the spectra are presented after baseline correction. The photoluminescence spectra of the prepared samples were obtained using a Spex-Fluorolog DM3000F spectrofluorimeter with a 450W xenon flash lamp as the exciting source. The excitation and emission slit widths were fixed at 0.5 and 1 nm, respectively, and a filter of 550 nm was used for taking the excitation spectra of samples. The microchemical analysis was performed on an energy dispersive spectrometer (EDS, OXFORD instruments, UK) coupled with an environmental scanning electron microscope (ESEM, FET-Quanta, FEI Netherlands). The selected-area electron diffraction (SAED) patterns and high-resolution electron microscopy of the samples were taken using a TECNAI 30G2 S-TWIN transmission electron microscope (FEI, The Netherlands) operating at 300 kV. A small amount of finely powdered sample is dispersed in an acetone medium by ultrasonication, drop-casted onto carbon-coated copper 13305

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grids using micropipet, and dried. All measurements were carried out at room temperature.

3. RESULTS AND DISCUSSION 3.1. Red Emitting CaGd1−xSnNbO7:xEu3+ Phosphors. Powder XRD patterns of Eu3+-doped CaGd1−xSnNbO7 samples shown in Figure 1 exhibit the characteristic pyrochlore peaks.

Figure 2. Excitation spectra (λem = 612 nm) of CaGd1−xSnNbO7: xEu3+ (x = 0.03, 0.05, 0.10, 0.15 and 0.20). The broad band from 260 to 360 nm corresponds to metal−oxygen charge transfer. The sharp peaks beyond 360 nm arise from intraconfigurational f−f transitions of Eu3+. The f−f transition peaks are less intense compared to the broad charge transfer band and are saturated for 15 mol % doping.

less electronegative Ca and Nb ions compared to Gd and Sn to the crystal lattice can result in CT band shifting to lower energy. Other sharp peaks in the excitation spectra correspond to characteristic f−f transition peaks.9 Of these, the peaks at 395 and 465 nm are of particular interest as these correspond to emission wavelengths of near UV and blue light LEDs. Figure 3 shows the emission spectra of CaGd1−xSnNbO7:xEu3+ (x = 0.03, 0.05, 0.10, 0.15, and 0.20) under 395 nm excitation. In the emission spectra, the hypersensitive 5D0-7F2 ED transition around 612 nm dominates over the MD transition. Hypersensitivity depends on site symmetry of lanthanide ion and the position of charge transfer band.28 Here, Eu3+ ions replace Gd3+ ions in the A-site which is

Figure 1. Powder XRD patterns of CaGd1−xSnNbO7:xEu3+ (x = 0.03, 0.10, and 0.20) calcined at 1300 °C (6 h) and Gd2Sn2O7:0.2Eu3+. All the peaks can be well assigned to a cubic pyrochlore type structure with a space group Fd3̅m. Eu3+ doping does not change the phase purity since there are no additional peaks with increase of Eu3+ doping concentration. CaGd1−xSnNbO7:xEu3+ phosphors have intense (111) pyrochlore superlattice peaks in comparison with Gd1.8Sn2O7:0.2Eu3+.

As the ionic radius and valency of Gd matches with that of Eu effective doping is possible. Absence of impurity peaks along with a small blue shift of (222) peak confirms this. Further, an interesting observation was the increased intensity of characteristic superstructure peaks in CaGdSnNbO7 in comparison with Gd2Sn2O7. Such variation in the intensities of superstructure peaks is associated with the variation in the oxygen position parameter. The elemental analysis using EDS (see Figure S1 in the Supporting Information) confirms the presence of expected elements and the obtained chemical compositions were close to the theoretical stoichiometry. The excitation spectra of CaGd1−xSnNbO7: xEu3+ (x = 0.03, 0.05, 0.10, 0.15, and 0.20) shown in Figure 2 has a broad charge transfer (CT) band from 260 to 360 nm centered at 306 nm. This broad CT band is due to the merged effect of O2p → Eu4f and O2p → Nb4d charge transfer transitions. The CTB of Eu3+ generally lies in the range 250−300 nm and the energy for this process depends on the covalency of the Eu−O bond and the coordination environment of Eu3+. NbO6 groups have a strong absorption in the 250−310 nm regions (see Figure S2 in the Supporting Information). It can be assumed that the observed CTB in the present excitation spectra corresponds to the CT transitions from O2− to Eu3+ and O2− to Nb5+. In complex hosts, the contributions from more than one component in the CTB cannot be resolved due to spectral overlap.1,26 For Gd2Sn2O7:Eu3+, the CT band locates at higher energy around 276 nm.20,27 As CT band energy depends on the electronegativity difference between ligand and metal,7 substitution of

Figure 3. Emission spectra of CaGd1−xSnNbO7: xEu3+ (x = 0.03, 0.05, 0.10, 0.15, and 0.20) under 395 nm excitation. As 5D0-7F2 electric dipole transition intensity dominates over magnetic dipole transition, Eu3+ sites have no inversion symmetry. After 15 mol % doping concentration quenching occurs. The inset shows the enlarged view of MD transitions. For low Eu3+ doping splitting of 5D0-7F1, magnetic dipole transition is not observable. 13306

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information on local ordering, distortion of site symmetry, etc. Raman spectroscopy is very useful in identifying vibration modes (phonons) in solids. The ideal cubic pyrochlore structure belongs to Fd3̅m (Oh7) space group. The site symmetry is D3d for A and B ions, C2v for the O ions, and Td for the O′ ion. According to group theoretical analysis, the 26 zone center vibrational modes possible are35

expected to have inversion symmetry. In the present pyrochlore phase the larger cations Ca2+ (1.12 Å, CN8), Gd3+ (1.05 Å, CN8) and Eu3+ (1.07 Å, CN8) occupy the A sites and the smaller Nb5+ (0.64 Å, CN6) and Sn4+ (0.69 Å, CN6) cations occupy the B sites of the lattice. The probability of nearest next neighbor for Eu3+ with one type of cation and two types of cations can be predicted like this: Ca2+-O2−-Eu3+-O2−-Ca2+; Gd3+-O2−-Eu3+-O2−-Gd3+; and Ca2+-O2−-Eu3+-O2−-Gd3+; Gd3+O2−-Eu3+-O2−-Ca2+; Ca2+-O2−-Gd3+-O2−-Eu3+; etc. The environment prevails for Eu3+ to the nearest neighbor changes with Ca2+ and Gd3+ either alternatively or one of the two cations, repetitively. With this kind of occupancy of three types of cations shared in the same lattice site, Eu3+ loses its site symmetry, thereby enhancing the hypersensitive transitions, 5 D0-7F0, 5D0-7F2, and 5D1-7F1. Usually, the site-to-site variation in the local structure around Eu3+ will result in broadening of 5 D0-7F0 transition. The broadening of the 5D0-7F0 transition in the emission spectra of CaGd1−xSnNbO7: xEu3+ directly proves the presence of Eu3+ luminescent centers having various local environments. Similar observations are made in Sr2−xCeO4:xLn3+ (Ln = Eu, Sm, Yb) phosphors.29 In the case of Eu3+ phosphors mixing of 4f6 configuration with the levels of opposite parity of the charge transfer state has high probability because the 4f55d state is at very high energy.7 So in Eu3+doped compounds, the energy of charge transfer states can influence the luminescence properties.30 This type of effect is more visible when these compounds are excited under CT band excitation itself as intense 5D0-7F0 bands. Figure S3 in the Supporting Information shows the emission spectra CaGd0.8SnNbO7:0.2Eu3+ under 306 nm excitations. As the CT band energy is low, the 5D0-7F0 transition can borrow intensity from the 5D0-7F2 transitions.31 In the present CaGd1−xSnNbO7:xEu3+ phosphors, the distortion of the A site symmetry and red shift of charge transfer energy leads to intense 5D0-7F2 hypersensitive ED transitions under 395 nm excitation. In pyrochlore phosphors, the 5D0-7F1 MD transition is expected to split into two. One corresponds to the A1g-A2g transition allowed in the z-direction (corresponds to [111] crystallographic direction) and the other A1g-Eg transition is allowed along both the x and y directions.4 Because the latter is allowed along two directions, in the emission spectra of pyrochlore phosphors one of the split 5D0-7F1 band has double the intensity than the other and the intense band is usually located in the shorter wavelength.3,5,32−34 In CaGd1−xSnNbO7:xEu3+ phosphors, the 5D0-7F1 MD transition splits into two for x ≥ 0.1 with almost comparable intensity and the intense peak is located in the higher wavelength region. It clearly indicates the deviation of pyrochlore A-site symmetry from a perfect inversion center and also the distortion of the scalaenohedra. Changes in the (111) super lattice peaks of CaGd1−xSnNbO7: xEu3+ in comparison with Gd2−xSn2O7:xEu3+ are clearly observable in the XRD patterns shown in Figure1. The increase of (111) superlattice peak intensity is due to the decrease in O48f positional parameter “x”. As the “x” value decreases, A site scalenohedra becomes more distorted. 5D0-7F1 MD transition splitting is observable for x ≥ 0.1. It may be an indication of ordering in the pyrochlore structure. The highly intense superlattice peaks in XRD patterns suggests long-range ordering in crystal structure. Sometimes the cation substitution can cause cation/anion ordering in pyrochlore lattice. As XRD is less sensitive to anion ordering, Raman spectrum can be used as a complementary analytical tool that can give additional

Γ = A1g + Eg + 4F2g + 8F1u + 3A 2u + 3E u + 2F1g + 4F2u

Here A1g, Eg, and 4F2g are Raman active, 8F1u is infrared active, and the rest 2F1g, 3A2u, 3Eu, and 4F2u are optically inactive. For pyrochlore structure, the Raman modes involve the movement of oxygen ions only while some IR modes involve motion of the A and B ions. For pyrochlore oxides, higher-frequency bands are due to B−O stretching vibrations, midrange bands are due to O−B−O bending modes, and sometimes A−O′ stretching modes may appear as lower-frequency vibrations.36 Raman spectra of CaGd1−xSnNbO7:xEu3+ samples are shown in Figure 4. The Gaussian fitting of Raman spectra was done (see Figure

Figure 4. Raman spectra of CaGd1‑xSnNbO7:xEu3+ phosphors. The ideal cubic pyrochlore structure belongs to Fd3̅m (O7h) space group have 6 Raman active modes (A1g + Eg + 4F2g). All peaks of CaGd1−xSnNbO7:xEu3+ correspond to characteristic pyrochlore vibrational modes. With progressive Eu3+ doping the peak around 320 cm−1 associated with F2g and Eg bending modes resolves into two separate peaks because of ordering in the crystal lattice. The mode appearing beyond 800 cm−1 may be due to the distortion of BO6 octahedra and subsequent relaxation of the selection rules.

S4 in the Supporting Information) and the resulting vibrational frequencies and tentative assignments of modes are given in Table 1. As Eu doping reaches 10 mol % the broad mode around 320 cm−1 resolves into two separate peaks. Long-rangeordered pyrochlore compounds can have short-range disorder because of the presence of vacancy or defects, which leads to broadening of peaks.37 Appearance of narrow peaks in Raman spectra points toward long-range order in the crystal lattice with progressive europium doping. This behavior is in agreement with the photoluminescent studies where the splitting of MD transition peaks is observed for higher europium doping concentrations. Because Eu3+ photolumines13307

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Table 1. Raman Mode Frequencies and Typical Modes of CaGd1−xSnNbO7:xEu3+ Phosphors cm−1 x = 0 x = 0.03 x = 0.05 x = 0.1

x = 0.15

typical modes48−51 Eg mode due to BO6 bending F2g mode due to BO6 bending F2g mode mostly due to B−O stretching with contributions from A-O′ stretching and O−B−O bending* A1g mode due to O−B−O bending F2g mode due to B−O stretch

314

316

313

318

316

349

349

345

343

340

419

418

417

417

419

508

507

508

505

508

604

600

608

606

607

*

The one and only mode related to the oxygen ions at the 8b site. All other modes are related to 48f site oxygen anion.48 The other highfrequency modes may be due to combination bands or overtones.35 Figure 5. Powder X-ray diffraction patterns of Ca1−ySryGd0.9SnNbO7:0.1Eu3+. For Sr = 0.3, additional impurity peaks appear in the XRD patterns.

cence spectra is highly sensitive to its local environment, it is effectively used here as a probe to find the short-range disorder within the pyrochlore lattice. Besides Raman spectra analysis, this can be further supported by the variation of asymmetric ratio with europium concentration. The asymmetric ratio (intensity of 5D0-7F2/intensity of 5D0-7F1) gives some clue on the symmetry of the Eu3+ environment. In the present system, the asymmetric ratio increases with the europium concentration (see Figure S5 in the Supporting Information). As always, disorder can give the appearance of higher symmetry, which is true in the present case for low concentrations of Eu3+.38,39 The short-range disorder for low concentrations of Eu3+ improves the symmetry, which can slightly deviate from D3d symmetry of the structure that lead to no splitting up of the 5D0-7F1 transition. From the emission spectra shown in Figure 3, with increase in europium doping the intensity of ED transition increases and reaches a maximum for 15 mol % doping after that concentration quenching occurs. As the europium ion concentration increases, the probability for resonance energy transfer between europium ions also increases and this leads to trapping of energy by defects in the crystal and hence in nonradiatve energy emission. 3.2. Influence of Sr Substitution on the Optical Properties of Ca1−ySryGd1−xSnNbO7:xEu3+ Phosphors. As Eu3+ emission spectrum can reflect any symmetry or covalent changes within the structure we have used Sr2+ as a network modifier ion. Strontium substitution cannot make any significant changes in the crystal structure but of course it will have some influence on the A−O8b chains as it is bigger compared to calcium ions. Figure 5 shows the XRD patterns of Sr-substituted phosphors for 10 mol % Eu3+ doping. Solubility limit of Sr ions in place of Ca was found to be 20 mol % after that some impurity peaks are observed. To understand the extent of modification of the lattice with Sr substitution, we carried out Rietveld analysis. Figure 6 shows the typical best fit including observed, calculated, the difference diffraction profiles, and the Bragg positions for Ca 0.8 Sr 0.2 Gd 0.9 SnNbO7:0.1Eu3+. Lattice constant, bond distances, and A−A distances of Ca1−ySryGd0.9SnNbO7:0.1Eu3+ phosphors along with R-factors are given in Table 2. The values of R-factors seem to indicate a reliable structure model. As the refinement yielded, strontium is expected to occupy the calcium site based

on its ionic size and charge neutrality. Lattice parameter and A−O8b bond distance increase with the increase in strontium substitution. The expansion of the crystal lattice with Sr substitution also results in an increased A−A distance. Accommodating cations having different valency and ionic radii in the pyrochlore A site can cause changes in luminescence properties. From the excitation spectra of Ca1−ySryGd0.9SnNbO7:0.1Eu3+ shown in Figure 7, the intensities of the CT band vary nonuniformly. The enhanced f−f transition intensities are due to increased polarization and distortion in the lattice because of bigger ion substitution.40,41 As the Pauling’s electronegativities of Ca and Sr are comparable (1 and 0.95), the CT band positions does not show any significant change. The emission spectra profiles of the Sr substituted phosphors under 395 nm excitation are identical with that of CaGd1−xSnNbO7:xEu3+, but emission intensities are enhanced with strontium substitution (Figure 8). By keeping Sr substitution at 20 mol % calcium, we varied the Eu3+-doping concentration up to 50 mol %. Electron diffraction pattern from a single particle of CaGd0.6SnNbO7:0.4Eu3+ phosphor shown in Figure 9a is consistent with pyrochlore-type unit cell. The pattern is indexed using the ratio method. From high-resolution TEM images shown in Figure 9b, interplanar spacing is 0.617 nm, which matches with the (111) plane of pyrochlore. The HRTEM and SAED pattern confirm the phase purity of CaGd0.6SnNbO7:0.4Eu3+ phosphor. Ca0.8Sr0.2Gd1−xSnNbO7: xEu3+ (x = 0.1, 0.2, 0.3, 0.4, and 0.5) phosphors show intense red emission and concentration quenching was observed at 40 mol % Eu3+ doping (Figure 10). Without Sr substitution we observed quenching at 15 mol % Eu3+ doping. With Sr substitution, the pyrochlore lattice constant increases, which can change the optimum Eu concentration, but such a large change in quenching concentration is unexpected. Eu3+ cluster formation can decrease Eu−Eu distance, which in turn will promote resonance energy transfer between excited Eu3+ ions and leads to quenching of luminescence. Of course, if such a type of quenching mechanism dominates, it will reflect in the lifetime values also. From decay curves shown in Figure 13308

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Figure 6. Observed (points), calculated (continuous line), and difference Iobsd − Icalcd (bottom line) profiles obtained after Rietveld refinement of Ca0.8Sr0.2Gd0.9SnNbO7:0.1Eu3+ using GSAS program. The powder data was refined using the space group Fd3̅m (No. 227) of pyrochlore structure and the corresponding atomic position set. First, the scale factors, zero correction, and background parameters were refined, with the cell parameter, 48f oxygen positional parameter (x), full width half-maximum (fwhm) parameters, and isotropic displacement factors being refined next. An asymmetry correction was also applied. The profile was fitted using pseudo-Voigt profile function. Vertical bars in the figure indicates the pyrochlore Bragg reflections.

Table 2. Room-Temperature Structural Parameters of Ca1−ySryGd0.9SnNbO7:0.1Eu3+ composition

y=0

y = 0.1

y = 0.2

y = 0.3

lattice constant, a (Å) oxygen x parameter A−O48f (Å) A−O8b (Å) B−O48f (Å) A−A Rp (%) Rwp (%) χ2 (%)

10.44628(26) 0.3172(10) 2.657(7) 2.26169(4) 1.975(4) 3.69332(9) 10.29 7.97 1.09

10.45409(28) 0.3216(9) 2.626(7) 2.26338(5) 1.994(4) 3.69608(10) 10.51 8.26 1.162

10.46439(32) 0.3203(9) 2.638(7) 2.26561(5) 1.9907(34) 3.69972(11) 10.29 8.02 1.089

10.47144(12) 0.3179(9) 2.657(7) 2.26713(2) 1.9830(33) 3.70221(4) 10.22 7.82 1.096

Figure 8. Peak intensities of 612 nm ED transitions of Ca1−ySryGdSnNbO7:0.1Eu3+ (y = 0, 0.1, 0.2, and 0.3) under 395 nm excitation. Intensities get enhanced with progressive Sr substitution.

Figure 7. Excitation spectra of Ca1−ySryGd0.9SnNbO7:0.1Eu3+ (y = 0, 0.1, 0.2, and 0.3) monitored for 612 nm emission. With Sr substitution charge transfer band center does not show any significant change but the 7F0-5L6 transition dominates over CT band intensity.

increases, which leads to the trapping of excitation energy by some killer sites and leads to increased nonradiative transition rates and hence quenching in luminescence. Because no intermediate ladder levels are possible in between the 5D0 and 7FJ levels of Eu3+ ion, the above-mentioned energy trapping is the only possible reason for quenching of luminescence. Concentration quenching can be increased if some big anion groups are incorporated into a matrix in which the Eu3+ ions do not share any O2− ions.42 Pyrochlore structure can be treated as

11, we obtained the lifetimes of Ca1−ySryGd0.9SnNbO7:0.1Eu3+ (y = 0, 0.1, 0.2, and 0.3) 5D0-7F2 red emission as 0.63, 0.61, 0.62, and 0.62 ms, respectively. There is no significant change in the lifetime values with Sr substitution. Exchange interaction between Eu3+ is of great probability for compounds having Eu separation below 5 Å.9 As Eu3+ doping concentration increases, the rate of exchange energy transfer 13309

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Figure 9. Selected area electron diffraction patterns of (a) CaGd0.6SnNbO7:0.4Eu3+ and (b) its high-resolution TEM image. SAED patterns can be indexed according to the pyrochlore unit cell. The d spacing of HRTEM is matching with the (111) plane.

Figure 10. Peak intensities of 612 nm ED transitions of Ca0.8Sr0.2Gd1−xSnNbO7:xEu3+ (x = 0.1, 0.2, 0.3, 0.4, and 0.5) under 395 nm excitation. The phosphors show concentration quenching beyond 40 mol % Eu3+ doping.

Figure 11. Life time decay curves of 5D0-7F2 transition (wavelength 612 nm) in Ca1−ySryGdSnNbO7:0.1Eu3+ (y = 0, 0.1, 0.2, and 0.3) under 395 nm excitation. Decay curves can be well fitted with a single exponential decay function y = Ae(−x/t), where “A” is a constant and “t” is the decay time.

interpenetrating networks of B2O6 and A2O′ units as shown in Figure 12. The A−O′ linkages form zigzag chains and two such chains intersect at O′. In the present quaternary pyrochlore oxides, there are three types of cations in the A site and we are doping Eu for Gd sites, which decreases the possibility of oxygen ions (O′) sharing by Eu3+ ions. The incorporation of the big Sr atoms to the lattice can then block resonance energy transfer between Eu3+ neighbor ions. The combined effect will be an increased quenching concentration. 3.3. Role of Effective Positive Charge of Eu3+ Ions in Ca0.8−xSr0.2GdSnNbO7+δ: xEu3+ Phosphors. The role of effective positive/negative charge of Eu3+ ions under CT band excitation are well-investigated and explained using the configurational coordinate model. It is reported that effective positive charge of Eu3+ ions will shrink the ground state CT band, which will increase nonradiative crossover between excited and ground-state CT levels and this results in less

efficient phosphor materials.43,44 But to the best of our knowledge, role of effective positive charge of Eu3+ ions on the luminescence properties under f−f excitation has not been reported. The Ca0.8−‑xSr0.2GdSnNbO7+δ:xEu3+ offers the possibility of doping Eu3+ ions to the Ca site also. The reduction of Ca amount for Eu substitution changes the crystal field but not the lattice site in the Ca0.8−xSr0.2GdSnNbO7+δ:xEu3+. The XRD patterns of Ca0.8−xSr0.2GdSnNbO7+δ:xEu3+ (Figure 13), show all the pyrochlore characteristic peaks. Above 20 mol % doping, additional impurity peaks were observed in the XRD patterns. The lattice parameter decreases with increase in Eu doping up to 15 mol %, which is evident from the shift of the (222) peak toward the right and after that peak shift toward the left can be observed. As Eu3+ replaces Ca2+ having higher ionic radii, the lattice value should decrease with the increase in doping. Such anomalies in the XRD pattern may be due to the 13310

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Figure 12. Schematic representation of the pyrochlore unit cell viewed along the (044) direction. The structure contains A2O′ (O′ → O8b) and B2O6 (O → O48f) units. Two A2O′ chains intersect at O′. The A cations also bisects O48f−B−O48f chains.

Figure 13. XRD patterns of Ca0.8−xSr0.2GdSnNbO7+δ:xEu3+ phosphors synthesized at 1400 °C. Here Eu3+ is expected to replace Ca2+ ions. All diffraction peaks corresponds to characteristic pyrochlore crystal structure. It is clear that in the present pyrochlore phosphors europium activator site exchange is possible without any secondary phase formation.

Figure 14. Raman spectra of (a) Ca0.8Sr0.2Gd1−xSnNbO7:0.1Eu3+, (b) Ca0.8Sr0.2Gd1−xSnNbO7:0.2Eu3+, (c) Ca0.8−xSr0.2GdSnNbO7:0.1Eu3+, and (d) Ca0.8−xSr0.2GdSnNbO7:0.2Eu3+ phosphors. Relative intensities of A1g (∼500 cm−1) and F2g (∼320 cm−1) vibrational modes get inverted when Eu3+ ions replaces Ca2+ ions. fwhm of F2g vibrational modes gets decreased as Eu3+ ions replace Ca2+ ions in the crystal lattice, which indicates ordering in the pyrochlore lattice.

interstitial oxygen ions in the crystal lattice for maintaining the electrical neutrality of the system. The EDS analysis (see Figure S6 in the Supporting Information) identifies the expected elements and the obtained stoichiometry composition is close to the experimental one. As powder XRD patterns are less sensitive to small variations in the oxygen occupancy, we have carried out Raman measurements. From Raman spectra, Figure 14, doping of Eu3+ ions to the Gd site does not make any significant changes in the relative intensities of Raman modes as both Eu3+ and Gd3+ ions have comparable ionic radii, electronegativities and

molecular weights. In Ca0.8Sr0.2Gd1−xSnNbO7:xEu3+ intensities of O48f−(Sn/Nb)−O48f bending A1g (∼500 cm−1) vibrational modes are higher than (Sn/Nb)−O48f(Sn/Nb) bending F2g (∼330 cm−1) modes. But the intensities of these modes get inverted when Eu3+ ions replaces Ca2+ ions. From Figure 12, as A cations bisects the B−O48f−B chains in the unit cells, higher electronegative Eu3+ ions can pull electrons of O48f ions toward it more strongly than Ca2+ ions. This leads to a decrease in the B−O48f−B angle and increases B−O48f bond length.45 Such variations lead to changes in intensities of vibrational modes. 13311

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ratio of europium doped Ca 0.8−xSr 0.2 GdSnNbO 7+δ :xEu 3+ phosphors are low compared to Ca0.8Sr0.2Gd1−xSnNbO7:xEu3+ as given in Table 3. Incorporation of cations with different valency and electronegativity to the crystal lattice was the reason for distortion of A site of present pyrochlore system. Doping of Eu3+ ions to Ca2+ sites will decrease the amount of divalent cations in the crystal lattice, which can cause an increase in the lattice symmetry. This is reflected in the emission spectrum where the asymmetric ratio decreased with Eu3+ doping. As both calcium and gadolinium go to the same A site, we do not expect any significant change in emission spectrum. Eu3+ in the site of Ca2+ ions experiences more positive charge and there will be some red shift in the CT band maxima.24 In the present system, the CT band maxima shows a small red shift to 309 nm from 307 nm as Eu3+ goes to Ca2+ sites instead of Gd3+ sites. However, the change in CTB energy is not significant on substitution of Eu3+ in the system, as it is nullified by the decrease in covalency because of reduction in the bond distance (As the incorporation of Eu3+ for Ca2+ leads to a decrease in bond length based on ionic size consideration24). But the most observable change in the excitation spectra is the increase of (f−f transition peak intensity)/(CT band peak intensity) from 0.78 to 1.68. When Eu3+ (ionic radius 1.066 Å, CN8) replaces Ca2+ (ionic radius 1.12 Å, CN8) in the crystal lattice, the cationic strength (charge/ionic radius) of the A−O′ chains increases. The trivalent europium ions can attract the electron cloud of oxygen ions strongly in comparison with divalent calcium ions. As a result, polarization of europium octahedral will be different in Ca0.8−xSr0.2GdSnNbO7+δ:xEu3+ in comparison with Ca0.8Sr0.2Gd1−xSnNbO7:xEu3+, and the mixing up of oxygen p orbitals with the f orbitals increases leading to the relaxation of parity selection rule.46,47 This contributed for the increased ED transition intensities of Ca0.8−xSr0.2GdSnNbO7+δ:xEu3+. A detailed comparison of luminescence properties of all systems are given in Table 3.

Figure 15 shows the excitation and emission spectra of Ca0.8−xSr0.2GdSnNbO7+δ:xEu3+ phosphors. One of the remark-

Figure 15. (a) Excitation and (b) emission spectra of Ca0.8−xSr0.2GdSnNbO7:xEu3+ phosphors. The excitation spectra show intense 7F0-5L6 transitions. In the emission spectra, intensities of both 5 D0-7F2 electric dipole transition and 5D0-7F1 magnetic dipole transition get enhanced. Asymmetric ratio of Ca0.8−xSr0.2GdSnNbO7:xEu3+ phosphors are less than that of Ca0.8Sr0.2Gd1−xSnNbO7:xEu3+, which suggests that as Eu3+ replaces Ca2+ ions, the symmetry of A-site of pyrochlore improves. The enhanced intensity is attributed to change of polarization of europium ions as activator site exchanges.

4. CONCLUSIONS A series of new stannate pyrochlore-based red phosphor materials: CaGd 1 − x SnNbO 7 :xEu 3 + , Ca 1 − y Sr y Gd 1 − x SnNbO7:xEu3+, and Ca0.8−xSr0.2GdSnNbO7+δ:xEu3+ were prepared by the conventional solid state route. Though intense superlattice peaks in the XRD patterns of CaGd1−xSnNbO7:xEu3+ phosphors assign them an ordered pyrochlore structure, the absence of 5D0-7F1 MD splitting for low Eu3+ doping is an indication of short-range disorder in the lattice and was further confirmed by Raman analysis. By using Sr as a network modifier ion in place of Ca, though the CT band position does not show any significant change the 7F0-5L6 and 7F0-5D2 transition intensities get enhanced. Substitution of bigger cations to the A site of pyrochlore reduces resonance

able changes in the excitation spectra is the enhanced f−f transition intensity. Phosphors show excellent red emission intensities and it increased with increase in doping concentration. The enhanced ED transition intensity arises from the symmetry distortion of Eu3+ in most cases. The asymmetric

Table 3. Excitation and Emission Parameters and Life Times for Different Eu Site Substituted Phosphors compd CT band position peak intensity of CT band ((peak intensity of 7F0−5L6)/(peak intensity of CT band)) (λem = 612 nm) ((intensity of 7F2)/(intensity of 7F1)) (λexc = 395 nm) peak intensity of 612 nm red emission (λexc = 395 nm) lifetimes of 5D0-7F2 emission (λexc = 395 nm)

CaGd0.9SnNbO7:0.1Eu3+ Ca0.8Sr0.2Gd0.9SnNbO7:0.1Eu3+ Ca0.7Sr0.2GdSnNbO7+δ:0.1Eu3+ 307 nm 3.6 × 105 0.6

307 nm 4.1 × 105 0.8

309 nm 8.5 × 105 1.7

5.5 2.5 × 105 0.63 ms

5.4 3.9 × 105 0.62 ms

5 16.8 × 105 0.64 ms

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energy transfer between excited Eu3+ ions and leads to an increase in concentration quenching. When europium is doped for the calcium site, the excitation and emission intensities are significantly increased. When europium replaces calcium in the crystal lattice, the mixing of CT band with f orbitals of Eu3+ leads to the relaxation of parity selection rule. This contributed for the increased ED transition intensities of Ca0.8−xSr0.2GdSnNbO7+δ:xEu3+. The above results indicate that quaternary pyrochlore system offers a lot of possibilities to manipulate the photoluminescence properties.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

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* Supporting Information S

Additional figures (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Tel.: + 91 471 2515311. Fax: + 91 471 2491712. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K.M. acknowledges Council of Scientific and Industrial Research (CSIR), Government of India, for the financial support towards a senior research fellowship.

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